The subject invention relates to feeding of single sheets of paper or the like from a stack of sheets for further processing by folders, printers, copiers or the like. More particularly, it relates to detecting double feed sheets which occur when a sheet feeder fails to properly singulate sheets from the stack.
In printers, copiers, inserters, and similar such systems it is frequently necessary to singulate sheets from a stack of sheets for further processing by the system. Many mechanisms have been developed to perform this singulation function, and, in general, they are effective. A typical sheet feeder is disclosed in U.S. Pat. No. 5,104,112 entitled "Document Feeder Having Reversibly Positioned Direct Drive Separator Assembly Motor" which issued Apr. 14, 1992 to the assignee of the instant invention. However, inevitably such sheet feeders will fail and feed a "double" (i.e. two or more overlapping sheets). Such double feed sheets may jam in the system, requiring operator intervention to clear the jam. Perhaps more importantly, if the sheets contain information or are otherwise unique (e.g. return of canceled checks) then their destruction in a jam caused by a double feed may significantly interfere with operations.
For these reasons it is known to provide such systems with detectors downstream from the sheet feeder to detect double fed sheets before a jam and possible destruction of the sheets can occur. One known method is to use an optical system to measure the transparency of a sheet after it is fed from the sheet feeder. Another known method uses precise, sensitive mechanical switches to detect an increase in the thickness of a fed sheet. Both of these methods for detecting double fed sheets involve precise, difficult adjustments each time the type of sheet to be fed is changed.
It is known to use a Hall effect sensor in a sheet feeder to sense the thickness of a document. However, prior art systems using Hall effect sensors require extensive linkage and thus are very sensitive to fluctuations in the feed deck and to any system vibrations. The prior art Hall effect sensor systems also have a magnet to sensor gap which is very difficult to adjust. Accordingly, the instant invention provides a Hall effect double detect feed system which requires virtually no linkage, overcomes the prior art problems and has virtually no sensitivity to fluctuations in the feed deck or any system vibrations.